Coated articles which have a high resistance to spalling have general application in the aerospace industry and, in particular, are useful as a coated anode in a vacuum tube for generating X-rays. Vacuum tubes used for the generation of x-rays typically comprise a cathode which directs a stream of high-energy electrons upon a metallic anode. The interaction of electrons of the anode atoms and the high-energy electrons produces x-rays. Most of the energy from the high energy electron stream is converted to heat energy. Since the anode is essentially in a vacuum, the only significant means of dissipating heat from the anode is by radiation. Since more heat results as power of the electron beam is increased, the use of high power may cause excessive heating of the anode, particularly at the point at which the electron beam strikes the anode.
In response to the problem of over-heating of the anode at high power, a rotating anode has been developed. A rotating anode is typically in the form of a spinning wheel with a beveled edge. The electron beam is directed upon a target track on the beveled edge. As the anode rotates, the electron beam strikes a surface of the target track, thus dissipating the generation of heat over a larger surface. Typically, rotating anodes are made of a molybdenum alloy with a tungsten insert for the target track.
Rotating anodes have enabled production of x-ray tubes of significantly increased power; however, power output is still limited by the transfer of radiant heat from the anode, which is in large part determined by the thermal emissivity of the surface of the anode. In order to increase the radiant heat transfer, either one or both of the faces of rotating anodes have been coated with a high-temperature resistant coatings which increase the thermal emissivity of the coated surfaces. Typical coating materials are metal oxides, such titania, alumina, zirconia, stabilized zirconia compounds or mixtures thereof. Common coating materials include a titania/alumina mixture, or a calcia stabilized zirconia/calcia/titania mixture.
With the development of higher-power x-ray tubes which are operated continuously for a long period of time, for example, for computer assisted tomography (CAT) scanning equipment, the heat dissipation problem from the anode has become more severe, and thus a limiting factor in the tube design. Another design problem is due to the fact that the front face of a rotary anode generally is of a higher temperature than the back face, while the tube is operating. Therefore, it is typical commercial practice to coat only the cooler back face, since prior-art coatings have generally been found to either spall off of the hotter front face or cause arcing between the track and the coated area. The mechanism of arcing is not completely understood, but it is believed to relate to the evolution of gases from the coating, such as H.sub.2 and CO. Therefore, the high temperature properties of prior art coatings, e.g. spalling and gas evolution, have often prevented coating of the front face and thus limited the ultimate heat transfer rate from the anode.
A suitable coating material should have a high thermal emissivity, while being resistant to high temperatures, and resistance to thermal shock which may spall the coating from the anode surface. In addition, the coating material should have a minimum evolution of gas at the operating temperatures of the anode. Further, the coating should have a thermal conductivity sufficiently high such that the coating does not insulate the anode and significantly impede conduction of heat to the surface. More particularly, the coating should meet the following requirements; (1) the coating should have a coefficient of expansion similar to the substrate material, (2) there should be little or no diffusion reaction between the coating and the substrate, (3) the coating should have a very low vapor pressure at temperatures above 1100.degree. C., preferably about 1300.degree. C., and (4) the cost of the coating material should be reasonable.
Although prior art coated anodes have been successful at moderate operating temperatures in increasing the radiant heat transfer from anodes, there is a continuing need due to increasing power requirements in the art for an anode with high thermal emissivity at higher operating temperatures and for highly emissive coatings which do not spall or cause arcing at these higher operating temperatures during use of the anode.